Arrangement for selection and demodulation of electrical pulses



Dec. 6, 1949 Filed Jan. 16, 1945 YNCHRON/Z/N PULSE SOURCE EARP ARRANGEMENT FOR SELECTION AND DEMODULATION OF ELECTRICAL PULSES 10 Sheets-Sheet l UNBLOCK/NG SWO 'comvsc rnva. OSC/L LA TOR CIRCUIT A I I com/5c TING OSCILLATOR CIRCUIT B r-' CONNECTING OSCILLATOR CIRCU/T c f l CONNECTING 0S6/LLA TOR CIRCUIT 0 SYNCHRON/Z/NG PULSE PATHS Inventor CNHRLES Wmuen 5919p A ilor/wy SIGNAL PATHS Dec. 6, 1949 c. w. EARP ARRANGEMENT FOR SELECTION AND DEMODU LATION OF ELECTRICAL PULSES l0 Sheets-Sheet 2 Filed Jan. 18, 1945 1n vcnlor Cannes w nunrfA-"A AP Dec. 6, 1949 c. w. EARP 2,490,039

ARRANGEMENT FOR SELECTION AND DEMODULATION OF ELECTRICAL PULSE-S Filed Jan. 16, 1945 10 Sheets-Sheet s Inventor H LE5 Wluum 5/72? A [torn y Dec. 6, 194-9 c. w. EARP 0,03

Filed Jan. 16, 1945 F/GS.

Inventor C/mmzs Wrumm E19,?!

Dec. 6, 1949 w, EARP 2,490,039

ARRANGEMENT FOR SELECTION AND DEMODULATION OF ELECTRICAL PULSES Filed Jan. 16, 1945 10 Sheets-Sheet 5 Inventor (mamas Wmun 1592 2,490,039 ION l0 Sheets-Sheet 6 Dec. 6, 1949 c, w EARP ARRANGEMENT FOR SELECTION AND DEMODULAT OF ELECTRICAL PULSES Filed Jan. 16, 1945 nventor CHARLES Mum [PF/D ARRANGEMENT FOR SELECTION AND DEMODULATION OF ELECTRICAL PULSES 1O Sheets-Sheet 7 Filed Jan. 16, 1945 I n uentor Cnnmss WILL/RM 599p Dec. 6, 1949 c. w. EARP 2,490,039

ARRANGEMENT FOR SELECTION AND DEMODULATION OF ELECTRICAL PULSES Filed Jan. 16, 1945 10 Sheets-Sheet 8 F/G/O.

w 1'7 P. i

[mmnlor 1 (mamas MLum-q 592/ Dec. 6, 1949 c. w. EARP FOR SELECTION AND DEMODULATION OF ELECTRICAL PULSES ARRANGEMENT Filed Jan. 16, 1945 1.0 Sheets-Sheet 9 Q mi lrwenlur HnRLss WILLIHWEEPP Dec. 6, 1949 c. w. EARP 2,490,039

ARRANGEMENT FOR SELECTION AND DEMODULATION OF ELECTRICAL PULSES Filed Jan. 16, 1945 1O Sheets-Sheet l0 -0 W A llorney of the generated oscillations, for synchronising the said oscillations.

The invention will be illustrated and explained in terms of a number of embodiments with reference to the accompanying drawings in which Fig. 1 shows a block schematic circuit diagram to show the basic arrangement of the embodiments of the invention.

Fig. 2 shows a schematic circuit diagram of a simple embodiment employing a selective filter for selecting the fundamental component of the incoming pulse train, and employing only one gating valve;

Fig. 3 shows a less simple but more satisfactory arrangement employing two gating valves;

Fig. 4 shows a preferred modification of Fig. 3 employing three gating valves;

Fig. 5 shows pulse form diagrams used to explain the operation of Fig. 4;

Fig. 6 shows a schematic circuit diagram of a simple arrangement employing an oscillator to generate a sinusoidal wave synchronised by the incoming pulse train, and employing only one gating valve;

Fig. 7 shows an improvement of Fig. 6 employing two gating valves;

Figs. 8, 9, 11 and 12 show embodiments similar to Figs. 2, 3, 6 and 7 respectively, but with means to prevent hunting of the drift compensating means;

Fig. shows a modification of that part of Fig. 9 above the dotted line X-X, in which an oscillator is used instead of a filter;

Fig. 13 shows a modification of Fig. 12 adapted to select a pulse train designated by an identifying tone;

Figs. 14 and 15 show schematic circuit diagrams of two arrangements for by-passing a selective element;

Fig. 16 shows a schematic circuit diagram of an arrangement for compensating the phase change through a selective element;

Fig. 1'7 shows a modification of part of Fig. 16; and

Fig. 18 shows a block diagram of an arrangement for synchronizing a number of oscillators from different pulse trains on the same carrier.

Fig. 1 shows the basic'circuit arrangement for selecting a particular pulse train from a mixed incoming signal which arrives over conductor I. The signal is supposed to include a number of trains of pulses of various repetition frequencies from which it is desired to select one particular train, and there may also be other kinds of interfering signals. The pulses may be simple unidirectional pulses, or packets or bundles of high frequency waves. They are first applied to a gating circuit G0 which is adapted to be opened only when each of the desired pulses is due to arrive. The pulses which pass the gating circuit are applied to a device designated SWG and designed to produce a sine wave output at a frequency which is substantially the same as the repetition frequency of the received pulse train, either directly from the pulses applied thereto, or under control of these pulses.

The sine waves obtained from the output of SWG may now be applied to a non-linear network NLN adapted to produce a phase change depending on the amplitude of the sine waves; this network is only necessary in certain of the embodiments of the invention and its purpose will be explained in detail later. The sine waves then pass to a limiter L adapted to produce substantially rectangular waves from the sine waves.

4 These rectangular waves are then applied to a pulse generator PG adapted to produce short gating pulses similar in character to the input pulses applied at I. These gating pulses are fed back to the gating circuit GC and operate to open it at the right times for the incoming pulses. The pulses after having passed the gating circuit may be supplied at 2 to suitable apparatus (not shown) in which they may be demodulated or otherwise treated. Output pulses may also be obtained from other parts of the circuit, such as from the output of the pulse generator PG at 4.

In the detailed circuits which will be described below, the blocks shown in Fig. 1 will not always be separately identifiable since two or more functions may be combined in the same apparatus. Fig. 1 does however exhibit the main features of the arrangement of the invention.

In the description which follows, some of the difiiculties associated with the proper operation of the system will be explained. It will be seen that the gate through which the pulses pass is opened under the control of those which have already passed through and accordingly it will be evident that some means must be provided for starting the reception, since as so far described there is no reason why reception of the pulses should start at all.

As will be explained later, the device SWG may take one of two difierent forms; it may be either a narrow band-pass filter adapted to select the fundamental sine wave component of the pulse train and substantially to suppress all the harmonics, or it may be an oscillator adapted to generate a pure sine wave having a frequency substantially the same as the pulse repetition frequency, the oscillator being synchronised by the pulses which have passed the gate.

Fig. 2 shows details of one arrangement according to the invention. The gating circuit GC comprises a pentode valve V biassed to cut-off by means of a source Bl connecting the cathode to ground. Signals are applied at I to the control grid which is connected to earth through the resistance R3. The suppressor grid potential is controlled by the gating pulses from PG so as to unblock the valve at the proper times, as will be presently explained. The anode Circuit is coupled to the device SWG through a transformer T of which the secondary winding is tuned by the condenser C5 to the pulse repetition frequency. In this circuit the device SWG is represented by a narrow band-pass electrical wave filter. When the pulse repetition frequency is low, for example 1000 pulses per second, a very convenient form of filter comprises two coils LI and L2 coupled electromechanically by a steel tuning fork F, tuned to the pulse repetition frequency. As is well known, such an arrangement forms a highly selective filter which will pass with appreciable amplitude only frequencies very close to the natural frequency of the fork. This filter could, however, take any form suitable to the repetition frequency of the pulse train; it could, for example, be a conventional wave filter, or employ a piezo-electric crystal or other resonant system instead of the fork.

The coil L2 is tuned to the pulse repetition frequency by the condenser C4 and is connected to the limiter L designed according to any well known arrangement to convert the sine waves into substantially rectangular waves obtained at its output. It might consist for example of two tandem connected valve stages both of which have a small or zero biassing voltage. Partial squar- A QQS beforeand/or.afterqtheedeviceSWGeif they shoulds 5 ber-required.a

Thezoutput of the. limiter'lrisloonnectedto the: pulse ;generator PG which; consists in: *thisicasepi a pain-oi: difierentiating; condensers-resistance:clra

alternately .positive .and' negative pulsestfromnthem square waves in well known mannera Thanegae tive-zpulsesproduced across Ril, have no-effect; and

thegating pulses and ,are. appiiedathrough a larger; 1

condenseraCiito the SUPDITGSSOIE-gljidyflf: the waive:- Vla'zw-hiclrds connected to.;the=;cathode;by resis ance R4 eshunted'; by a rectifier. r:X;--; shown 351383 diode througha selenium :oiz:;copper oxidesortbe -useck instead.z

The l diode is connected ;with its anode :to, .the-.-- suppressor grid oithevalve V. Ifit .beaassumed; thatgthere are no, signalslapplied. 1at= l,; the sup saturation lofathesinon poresof L2 las' thecsamplitudeevariesrt Thisxcanab'earranged: tor produce;

a similarrcorrecting. phaseechange -,in::the=;output sine wavea Alternatively; anadditionalltunedzcire pull like. L23 C4 (not :shown could A'bE/PIOVidGdZ torepresent NLN; andzwould; operate in: the same.

W aye.

stilltwanothen methodzzis: .tozoverbias the .afirst v-alvestagm ofithe, limiter, L; irrisuch, ;a way... that.

0 .=.the;rnegative portionofl the squarerwave in :the. 1

eanodezcircuittisof smal'lerduration than the posi-.

tiveeportion;:;ror-:weaki signa1=s..; Fonstronger sigrials these :portionsrtend to:.become more equalin .7 durations.- Thislsw-ili evidentlywary the timing of; -the;-.gating;;pu1ses1 generated: by'.PG.fWith respect l L to the sinerwave output fromSWGn,

The natunalt state of- 'equilibriumeof .the system of Flgn2-rcorresponds;:;to theecondition in which:

eachincoming signalzpulse rideslon one edge :of: other kind of rectifying udevices.onrcircuiiacouldar the correspondingtfgating; pulse: Movement. of."

thersteep edgepf the gating. pulse applies .considerableiaautomatic gain .i-control: toxthe :valve. V rso 1 th atnthe signal levels.are-automatically adjusted:

so that. the desired-.coincidence of the; inputand pressor grid Willbe .maintained ;substantially;=at:..-. gatingepulsessis secured...

cathode; potential, so that the valve is in .a- 0112:

ditiomto. accept any; signals which: mayarrive; in other, wordsw'thetgatel-is .standing;;open-. Whem. signals-rare: applied-at ..l (which signals. include The'scomponentsrlzofiithe circuitx-Cl, Hi .can he selectedaisollthatthe; gating pulsesrare of. appropriate duration to unblock the valve V'-10ngs:

enough reliably; to ,accepteach 1: incoming. pulse.

allztheennwanted pulse trainsandotherinterfgenllikewisesthe:componentseCLsRl can be selectedv encel-a; the forkfi-lter will select the-fundamental. sine .wave :component fromethe Iwanted pulses, and. the egatingapulses will. then: ;he supplied to. the-z;- suppressorgrid: from PG.. onzaccount: of the pres-z ence of-zthe diode, these pulses build up;,a'.;ne g 3 tive: bias... potential: on, therrsuppressor grid,:'by: chargingthe .condensersC3; so that in: the inter? valsr between 313119: gating.-; pulses; the; valve: is blocked; However, just; at" theeperiods ofi;.the=;=v peaks ofgthese positive. pulses ,the.,=,suppressor grid- 1 0 potential 'issraised substantiallyto; that iof; the a cathode :so that the, valve 31s: unblockedv to v admits. the wanted pulses:

It::will. be: seen that-by; this-arrangement, ,the gateis'at signals,;--including the-:wanted pulse:trairrlaresre-e ceiyedi thergating pulses: cause;- it. to lbe -shutnto. all but-the wantedv pulseetraima It; will be evident. that the/gating .pl1 5es -must.:;

be properlytimed;to :unblock the yalve-atthe right q moments-sand the .arrangementmust not tendtoi get.;0ut ;0f.step.= It" is .=for: this; latter:pl rp0se that thanetwork'NLNiFig. 1); (units equivalentl may be :provided. ImFig l. of .but its function; may rbe; carried::out1.-:inl; several ways in the apparatus-already shown:

Thusgadvantage may .betaken ofsthezfachthat the;mean :di-rectsmagnetic attractionbetweennthe forkz'gF-yandnthel coils; Ll and-L2. amplitude .of avibration; and. this -:causes -a corree sponding; slight variation in;thie rnatnrahfrequency's of therforlaa. This "is accompanied by aciariationwz of therphase-v of .the sine-:wave outputtirelative :toz.

firstastanding open; ,buttasisoonsas :anys" 4 2 there .a'islno .separate zrepresentativer so that suitable pulses-:are obtained at1z4 for any otherpurpose;

Fig-.:3..-shows:a modification'of Fig. 2 in which twowgatingl valveseare provided, ad'aptedxto' he 5: l ened =atwslightly different .:times,: the wanted:

pulses :passing;mainly throughclone or'th'e other according; as they r are-late or 1 early. The gating times tendrtoldrifzt. automatically into coincidencewithztheetimes ofziarrivalpf. the pulses so that ;;they';ulthnately;pass =throughazboth the gates.-

The-513W!) gating valvesVA'and. VB are arranged in "the samewaywasttheavalve V1 in Fig. 2;.but sharera common'gri'd; resistance R3 and negative biasvsou-nceiBZa Thewathodes could alternatively hayezpbeerr vbiassedg; positively. Tl'lesuppressor grids are connectedx to-ithe.corresponding oathodesgthrough resistance .RAA and RAB shunted 'by rectifiersXA:andXB;.

Th'e device'-.=SWGimay Joe the s ame'as in Fig. 2," though'ras ,:already. mentioned-zany suitable type of zfilterscould be used. instead: ofa-ior-k: The dea vicar/SW6: obtains its zinput directly. from the transiormen'I'B connected ins-series with the anodeiwlrcuit of:VB, anduthrougha condenser C6 rrorn.thetransformer. .TA connected inseries with:

the anodeicircuit :pp VA. This ensures that there is a phase difference-ofs-substantially .90 between.

the-liundamental. components voltages applied to SWG: fronrftheetwoivalves: VA and VB, the reason variesrzzwith. the fonwhiclnwillbejexplainedzlater.

The'eoutputzofithe; device SWG feeds in paralleleithreerzlimiting valves :LYA,I.LVB-. and; LV.

Thesewalvesshare 1a.:common grid. resistance R5 and-'zhave. infiiyiduaLresistance RTA; R73 and R1 therinputapulsesn Should the gatingrpulses; tends. 55 m series with their control-grids. Theanodes are to drift out of coincidencmwith; therincomings; pulseszthe driving forcezonathe :fo'rlcds reduced:;.. andrits. frequency varies slightly}. and; ifiiiSLfOUlldi'i thaththis variationis in :the Vright;.directionz;to

respeetivelyrconnecte'd to the high tension; source HT+ through resistances RBAcRGBiand R6, As in :thercase ofeFig.:2Iappropriate amplifiers shown) .maygbezintroduced .before :and/orafter correct. the: phase of mes-gating pulses-a Thereri 7onthaadeviceaswfil is thuszno tendency-tenths:rgating pulsesitowane:

derout of step.

Another.rmethodr:ofiiachieving the:;.sameeresult:5;

is to stake advantageyofetheavarlationsrin ztuningn of :thescircuitiL't', p04; due; toLWariat-ions .sm:..theai75eare connectedarespecticelystorthe@suppressongrids Three :differentiating circuits R l A, C lA, R l B, C lBci zzandcRl-im :for producing short pulses are respectively;connected:toltheanodesv ofrthe-threelimitingayalves :LVA; LSZBrZ'and LV; the: first two- (not 1 of the gating valves VA and VB and the third to an output terminal 4. The cathode of the first limiting valve LVA is biassed slightly positively to earth by the source BA, and that of the second LVB is biassed slightly negatively by the source BB. The cathode of the third limiting valve is connected to earth. The slightly different bias Voltages applied to the three limiting valves cause the leading and trailing edge of the square waves respectively produced in their anode circuits to occur at slightly different times. On application of the signals at terminal I, the two gating valves are at first both open, and pulses get through to start up the fork in the device SWG. For small fork amplitudes, the derived gating pulses are broad and the two trains applied to the gating valves are appreciably staggered apart. As the fork amplitude increases, the gating pulses become shorter and sharper and the pulse trains close up more nearly into coincidence.

The phasing of the driving and output circuit of the fork is so arranged that when the signals first arrive, one series of eating pulses occur slightly before the pulses of t e Selec d incoming train and the other series occur slightly after. The third train of pulses from terminal 4 will be substantially coincident in time with the incoming pulses.

Thus the gating valve VA provides the early gate, and VB the late gate. The connections of the circuit are so poled that for whichever gate the incoming pulses tend to pass the result is to tend to shift the gating trains in such a manner that each of the incoming pulses is more symmetrically placed between the two corresponding gating pulses. The gating pulses close up together as the fork amplitude builds up, and the signal pulses remain securely meshed between the pairs of gating pulses until they are all practically coincident. The train of output pulses from the valve LV then coincides in time substantially with the incoming pulses, and these pulses may be used to open another signal gate by any suitable arrangement (not shown) for selecting the desired pulses, which may then be demodulated or otherwise treated in any other way.

The manner in which the arrangement operates will be more clearly understood by first assuming that the circuit is steadily receiving a train of the desired input pulses. As already stated, the incoming pulses are meshed between the pairs of gating pulses, so that they will pass partly through each of the gating valves, which are opened at slightly different times. Accordingly on account of the condenser C6 which provides a phase difference of 90 between the driving waves derived from the two valves, the resultant driving wave applied to the filter will be advanced in phase by 45 with respect to the wave derived from VB alone. A total retardation of phase of 45 is arranged to be produced in passing through the filter, and through the subsequent circuits terminating in the grids of the limiting valves LVA, LVB and LV. Now suppose that a small drift occurs tending to make the gating pulses a little too late. (Such a drift may occur in the circuit of Fig. 3 or in the incoming pulses themselves.) The pulses now tend to pass rather through VA, since this, being the early gate, is caused to open a little late. A small advance in the phase of the driving wave therefore occurs and this will advance the gating pulses slightly so as to correct the drift.

If the drift should be large enough to make the incoming pulses completely away from the con 7 ing valves respectively;

trol of the gating pulses so that they are too late or too early to pass through either of the gating valves, then the fork will lose its drive altogether and the amplitude will decrease, thus widening out and spreading apart the pairs of gating pulses, so that the incoming pulses will be eventually picked up again by one of the gating valves. The fork Will then receive a drive in such a phase as to close up and sharpen the gating pulses and to shift them so as to mesh the incoming pulses between them in the manner already explained.

A load resistance RL may be connected if desired, as shown, between the positive high tension terminal HT+ and the common point of the primary windings of the two transformers TA and TB. This enables the pulses which have passed through the gates to be obtained at terminal 5 connected to this common point through a blocking condenser K.

In Fig. 4 there is shown a preferred arrangement in many respects similar to Fig. 3, but there are in this case three gating valves, two of which are early and late gates respectively, corresponding to VA and VB of Fig. 3, but which are both initially shut and remain so except when opened by the gating pulses; and the third gate, represented by the valve V0 is initially open, but becomes shut as soon as signals arrive, in the manner explained with reference to Fig. 2, being opened periodically by a third train of gating pulses. This may be called the normal gate. When the incoming pulses are being steadily received, they pass through all three of the gates.

The gating valve VC is arranged similarly to Fig. 2, and has its suppressor grid connected to the oath-ode through a resistance R40 in parallel with a rectifier XC, but the early and late valves VA and VB have no rectifiers. The three cathodes are biassed positively by the source B3 so that the valves are cut off. The three control grids share the grid resistance R3 and are all connected to the input terminal.

The device SWG may be as before, amplifiers (not shown) being provided if necessary. Driving voltage is obtained from the valve VC through a transformer TC tuned With a condenser C50, and through the condenser 06 as previously used; and also from the transformer TD tuned with the condenser C5D to the primary winding of which are connected the anodes of VA and VB in opposition, anode current for these valves being supplied from the terminal HT+ through the centre point of the primary winding of TD. Thus the voltages derived from VA and VB are advanced and retarded respectively by with respect to the voltage derived from VC.

Three limiting valves are provided in cascade, of which the first, LVC is driven from the output of the device SWG and produces steep trapezoidal Waves. These are applied in turn to LVA and LVB where substantially rectangular waves differing in phase by 186 are respectively produced. Pulses derived from LVC by difierentiation in the circuit REC, Cs C are applied through condenser 03C to the suppressor grid of VC. Special unblocking pulses are applied to the suppressor grids of VA and VB from LVA and LVB in a manner which will be explained later. Short output pulses may be obtained at terminal i from the valve LVB by difierentiation in the circuit R2, C2 and may be applied for purposes previously explained. The resistances RSA, RSC are anode supply resistances for the three limit- RSA and R53 are output loads forthe valves LVC and --LVA respectively, and RJA and RlB are grid series resistances for EVA-and LVB.

The operationof the circuit will be explained with reference to the diagrammatic wave-forms of Fig.5. The points at which the various waves occur are designated in Fig. l with the corresponding numbers of these curves. The incoming pulses at terminal I which it is desired to receive are shown at WI. The trapezoidal waves W2 are obtained from the anode of the first-limiting valve LVC from the sine wave output of the device SWG applied to the control grid of this valve. These, after differentiation in the circuit RIC, CIC, become moderately sharp pulses W3, alternately positive and negative, of which'the positive pulses are used to unblock the gating valve VC at the tips of the pulses in the manner explained =with'reference to Fig. 1. It will be understoodthat these pulses cause the initial blocking of this valve on the first arrival ofsignals as previous-1y described.-

The trapezoidal wavesWZ are also applied to the second limiting valve LVA, from the anode ot'which are obtained practically rectangular waves W4 'inopposite phase. These in turn are applied-to LVB, and produce from the anode another series of rectangular waves W5 in the same phase asWZ. The waves W2 from LVC are also applied in parallel :to the two differen-. ti'ating circuits RrlA, CIA and. B1B, CIB producing-pulses like W3, to which are added waves and. W5 respectively after appropriate at- -tenuation by the networks RBA, RIA and R813,

R113 respectively. The resulting waves W6 and- W I are. applied to the. suppressor grids of the gating valves VA- and VB. The Waves W6 will :be seen to consist of-positive peaks av separated .by-.horizontal portions from negative peaks which have no effectand are disregarded. The trail- .ingredges of thesetpeaks a are very steep, but

theleading edges are much less steep, being .similarto theleading edges of the pulses W3.

The waves Wlrlikew1se have positive peaks 1) corresponding to the .peaks at but with the leading, edge verysteep instead of the trailing edge. The ,steepness of these edges is enhanced by the condensers CTA and CTB respectively shunting theresistances BSA and B8B.

The valves VAand VB are respectively unblocked bythe positive peaks a and I). Since the leading edge of b is much steeper than that of a,= the gating valveVBwill be opened later than VA,, and so it will be the late gate. Likewise, the-gating valve VB will be shut later than VA. Tlregating pulses W6 and W1 are of course only produced when the. fork amplitude has become appreciable.

The stabilising action of the circuit may be considered by supposing that the incoming pulses are being steadily received. These pulses will .be passing through VC and also about equally through VA and VB. The component of the driving voltage derived through the transformer from these two valves will accordingly be practically zero since the valve anodes are connected in opposition to the primary winding. Practically all the drive therefore comes from the valve VC, and will, be advanced in phaseby about'90lon account of the condenser cc. If 31;!10W1b6 supposed that a driftoccurs tending to-make =the gating-pulses W6 and W1 relatively late,- the incoming pulses will tend to pass through the early-valve VA rather than through thelatevalveVB, so that -a driving componentto formthe'gating impulses as before.

the introductiqn. of arnplifiers before and after the device SWG- is not essential, it is usually preferable at least toinsert an amplifying valve before the device in order that the driving transformer may operage into a substantially infinite impedance Thiswill prevent the deviceSWG from placing a load on the transformer which might upset the desired, phase adjustments of h rcui a Fig. l 6] showsan arrangement employing. a single gating valve, but differing from Fig. 2 in that the device SWG is an independently oscillating generator,whichv is synchronised by the pulses which. it is .desired to receive. The circult does not, require any device corresponding to .the,network NLN shown in Fig. 1.

Referring to Fig 6, the gat ng valve B is biassed beyond-the cutoff of thesource B connected to the cathode. The suppressor grid does not in this case require arectifier, and the gate isinitiallyshut. R3 is .the grid resistance across which the incoming pulses are applied. The device. SWG. comprises .a valve VO arranged inia conventional oscillating circuit including the secondary winding of the transformer T tuned closely to thepulserepetition frequency by the condenser CO., R9 is the grid. resistance for-V0 and the condensers are the usual blocking condensers. The pulses-which pass the gating valve are fed through the primary winding of the--transforn1er;- I-to synchronise the oscillations of=the valver V0. These oscillations are applied across the outputlead resistance. to a limiting valve LV-wh-ich-provides nearly rectangular waves whichare;dif erentiated in the circuit RI, C1 B6 and R1 are the anode and grid series resistances for 14V.

The gating-pulses are supplied along the conductor 3 to thesuppressor grid of the valve vV and operate tosunblocka it just when the incoming pulses are-due to arrive.

It is necessary that the oscillation circuit should beatuned to. oscillate when uncontrolled at a, frequencywhich-isnot exactly the same as the. pulsarepetition frequency, but should be close-.enough thereto .to permit the incoming 'pulsesto exert-control. As already explained, the valve V is normally shut, but the free oscillations of the circuit generate pulses which periodically open the valve. When the desired pulse train first arrives, the valve V will in general notbe -open atthe right times to receive the pulses.- However; owing to the slight difference in frequency there'will be a progressive drift in the relative phases so that presently the incoming signals will be picked up and will pass through the gate. Theythen synchronise the oscillator so 3 that the proper phase relationship is maintained."-

It is to be noted thatwhen the pulses have passed through the gate they will tend to drive the valvevpin quadrature with the oscillations generated'thereby. If it be supposed that the '75 tuning of the oscillating circuit is such that there is initially a continuous retardation of phase of the phase of the oscillations, so as to correct the retarding tendency of the gating pulses. This may be done by appropriately poling the connections of the primary winding of the transformer.

In this way a stable arrangement is produced.

The pulses which have passed through the gating valve may be obtained from the terminal 2, or alternatively the gating pulses may be taken from terminal 4 and used in any desired manner.

In the arrangement of Fig. 6, it is only possible to correct a continuously retarding or a continuously advancing drift tendency, since it will be clear that the two cases require opposite polings of the primary winding of the transformer T. If it can be assumed that the uncontrolled frequency of the oscillator will remain always slight- H ly higher or always slightlylower than the recurrence frequency of the pulses, such a unidirectional tendency will be maintained. However, small changes in the operating conditions of the circuit may easily upset this condition, and accordingly the modification shown in Fig. '7 is preferable. This is like Fig. 3 in that two gating valves are used providing opposite compensating effects.

The arrangement is similar to Fig. 6 except that the two gating valves have their anodes connected in opposition to the primary winding of the transformer T. Gating pulses are supplied direct from the differentiating circuit to the suppressor grid of VA, but through a delay network DN to that of VB. This delay network should have a characteristic impedance equal to RI. The valve VA accordingly comprises the early gate and VB the late gate.

If it be supposed that the oscillator has drifted until the incoming pulses pass through VA, then a quadrature drive is applied to the oscillator, and the connections of the primary winding of T should be so arranged that the output phase 'of the oscillations is advanced. This will tend to make the gating pulses a little earlier so that the incoming pulses will now tend to pass through VB causing in turn a retarding tendency. It will be seen that the arrangement will stabilise so that the incoming pulses are meshed between the two series of gating pulses, and any tendency for a drift in either direction will be automatically counteracted.

Pulses which have passed through the gate can be obtained at terminal 5 connected through a blocking condenser to the load resistance RL connected in series with the anode voltage supply lead for the valves VA and VB and a derived pulse train coincident with the incoming train may be obtained from terminal 6 connected to a centre tap on the network DN.

'I he oscillator shown for illustration in Figs. 6 and 7 is of a very simple type. It will be evident that various other types could be used provided only that they are capable of control by the incoming pulses in the manner explained. Actually the simple oscillator shown may not be suificiently stable to allow a given train of pulses to be selected with certainty in the presence of others with closely adjacent repetition frequencies; Therefore a crystal or fork controlled oscillator, for example, having good stability may be more suitable. It may further be desirable to provide a Wave filter (not shown) between the gating valves and the oscillator circuit in order to prevent transient phase control of the oscillahunting effect whereby there is a continuous swinging backwards and forwards of the drift correcting operation. Thus any displacement of phase of the gating pulse trains produces an immediate corrective reaction on the waves supplied to the device SWG, whether it be a filter or an oscillator. The response to the corrective reaction is of course not instantaneous, and the corrective reaction persists until the proper phasing has been reestablished. At the moment when this occurs, however, the phase of the gating pulses may be changing rapidly and may overshoot the correct value before the reverse correction can take effect. In order to deal with wide ranges of phase change in filters or frequency drift in oscillators, so as to obtain a high precision of coincidence, it is necessary to use large quickly acting corrective forces, and the effect combined with lagging response may produce hunting, or continuous swinging of the control circuits. The circuits to be now described contain means for applying the corrective forces in such manner that they cannot alternate rap idly. There will then be no tendency for the response to lag behind the control, and hunting is therefore prevented.

Fig. 8 is a modification of Fig. 2 including means for preventing hunting. Details which have the same designations in the two figures are supposed to be similar and will not be again described. The drive for the device SWG is provided through a pair of amplifying valves Al and A2 having their anode circuits connected in parallel, the valve A2 being biassed beyond the cut off by the cathode source B3. This valve should preferably be of the variable gain type. Al is biassed for normal constant amplification by a conventional resistance shunted by a condenser in series with the cathode. Pulses which have passed through the gating valve V are applied directly to the control grid of Al and also through a small condenser C6 to the control grid of A2 so that the two valves are driven in quadrature.

A rectifier Q, which may be a diode or any other suitable rectifying device, is associated with resistance R10 and condenser CID to provide a unidirectional bias voltage for the control grid of A2 which is derived from the pulses transmitted through the transformer T. This bias is such as to make the control grid positive, so counteracting the positive cathode bias. The circuit L3, 09 is tuned to the pulse repetition frequency, and is provided to prevent the short circuiting to earth of the control grid for the signal waves.

It will be seen that when control pulses begin to pass the gating valve, the valve A2 is blocked. It becomes eventually unblocked, however, as the positive bias on the control grid builds up, and applies driving waves of increasing amplitude in quadrature with those passed by the amplifier Al to the device SWG. The resultant driving wave thus advances in phase until equilibrium is reached.

The time taken for this to occur depends on the time constant of the circuit RIO, cm, which may be quite large. Any subsequent drift in the gating pulses produces a change in the amplitude of the pulses passed by the transformer T. This change is not immediatelyeifective on the valve ama can A2 owing to-the. time. constantiofthe circuit. B10, C10, but a-gchangegin theydrive through'A2 Teyentually occurs, changing the iphase ofgathe resultant driving wave sothat the drift isoorrected. Owing to the lag introduced in the application of the correcting force, the circuit cannot hunt.

It "will-be understood, of course, that the net,- work NLN- shown in Fig. l is not required inthis modification, nor is thereintended to-be any ele- 1'21 isztnned-zto thesen-entice frequency bv the condenserflfini,

Flax-10. shows-amodification;of-the upper part ofiFig.'9,-f:aboueithe dottedline the leads 8 -proceedingadownwardsqin: athe. twonfigures, being idfilltlfifidibyithb same-.rlettersato e. The device SWG,:isenrosoillatonrlnsteadof a filter. As explainednwithireferences toeFig- 6, when, an oscillator cis-izuseds,thecgating; valves are initially merit in the circuit which; performs a similar L shut so that the elements XA and KB shown in function. The desired corrective phase change is produced by the, valve A2 in the manner explained. Apart from the lag introduced to correct the hunting, the circuitrof Fig. 8 operates otherwise similarly to Fig.2, and. equilibriurnc-oocurs when each incoming pulse coincides wit-hone edge of the correspondingly gatingpulse.

Fig. 9 shows, a modification of- Fig. 3introduoing means for preventing hunting similar to those described in connection'with Fig. 8. Elements which. are the, same in both figures are similarly designated and will not be again. described.

Thegating valves VA and VB are connected .to the device SW G by a pair of amplifying valves Al and A2 of. variable gain, and by a diiferential rectifying circuit comprising a pair of similar rectifiers QA and. QB of any suitable type, such asdiodes. The anodes of thetwo gatingvalves are connected together by a pair of equal resistances RI IA, and RI IB, the junction point of which is connected through theprimarywinding of the transformer TA to the high tension source at HT-lthrough a load resistance RL. Thus the res stances RI IA andRi lB form a, differential output, load for the gating valvesand the transformer forms a common load.

The differential rectifier circuit comprisesthe two rectifiers QA and QB, the resistances RNA and M35, and the condensers Cl IA. and Cl IB. This works into a pair of equal load resistances R 1 2A and R! 23 connected in series, with the common point connected to ground. Each. of these load resistances is shunted by a bye-pass. con,- denser K.

The amplifier valve Al is driven direct from the secondary winding of the transformer TA, and A2 is driven in quadrature therefrom through the condenser C6. The cathodes ofthese valves are'biassed by a conventional series, re sistance and condenser circuit, and the control grids are also variably biassed from the rectifier circuit, according to. the manner in which ,the pulses pass the gating valves. Thus, under normal conditions when equilibrium is establishedv the pulses will be passing equally through both 5 gatesandthe opposedoutput voltages across the resistances RI M and RI lB will be eoualfso that no "bias voltage "will be applied to either control grid.

Fig-1.9mm required and, should be omitted whenithe Fig: .l-Omnodification: is employed. An exactly'similamrectiwingscircuit is used,,but the values-1A1;andiAZrare now driven in opposition 1'5 fromrthertmnsiormet TA; Variable control grid bias appliedgfromzzthecrectifier circuit through the resistances: man-r and: RMB. The transformer TA is tuned to the pulse repetition frequ ncyabuitheicondensen'C5Aras before; the coni d'ensers are ablucking,voondensers.-

= .transformerrmr The; output. :of 'the device SWG controls? thelimiting: valves in. .the same way as infi igSLBvaI-idrM When: rstablse equlllbriunrz has been reached, Signals". pass: equaliu through: both the gating "valves: Thcrerwillzitherefore be no control grid blast-potential zd ueloped'r-br he rectifying circuit, so that both the valvesrAi :and-AZ-will have the same'igai-npandi theazoutput tending to control ;the oscillations thfiblighhthe transformer T 135 s.;t-hereiore.rzerohsineezthe itWO valves are in opposition; If:ragdriftaoccursrsorthat the incoming pulses,:tendrtocpassiratherthrough VB an VA, thetcontrohlgrideof iAZ .1::becomesrslight1y positive and-that .1of;5A-1 lslightlywnegatiye as a result of 4 .the reaction;-ofirtherrectifiencircuit, so that a small controllingmutputqwil-l be obtained whose phase is determinedzbycAl-g If the pulses tended fls passsthroughz VA instead; the phase of the controlling output would have been opposite, as

ldeterminedeby thewalue l 5 .thatthecontrolling;'waveacorrects the eiiect of the drift. 'Ihexlag producedby-the time constantiofqthe reotifier cigrcuit prevents hunting, as efore:-

Fig; li showsaamodlfication of'the arrange- 5.;men-t:-1ofi;-Fig-.-;6. ,Inzthispase the transformer T does not: 013m part pt. helcsci-llating circuit; but supp i swwavescwhlchia re tifi d y he ircuit includinse-the-rectffier Q, resistance RH), and condenser-11!,toeprovide a variable positive bias The valves A! and A2 will therefore have sub for-the: contrqlggrid of the amplifying valve A2 stantially equal gain. If, however,- a drift occurs such that'the pulses tend to pass rather through then the rectifier circuit will cause the-con trol grid of Al to become slightly positive, and

through atheetunedncircuit L 3, C9. The transduotanceL3 should be tuned by theihrrespectiy acondensersyto the pulse repetitionzfrcquency s before: These elements will be that of A2 to- .beoome slightly negative, so that $6.5 seen"toi besirrangedsinrsubstantiallythe same way the output of A! will exceed thatof'AZ, thus retarding the phase of the'driving'force acting on SWG, and so correcting the drift. An opposite drift produces the reverse effect, but as in asiinrFigiwiiz fThe oscillating-valve V0 has a conventional oscillating circuit LO; CO associated therewith;Jtunedrnearto the repetition frequency of-the pulses, and 'supplieswaves to the limiting the case of Fig. 8, the time constant of the recti- 730 1 1,y t1 1 as in fi,

fier circuit delays the applicationvof thecorrective force, thus preventinghunting as previously explained. It should benoted that the inductance GOilllLiB oorrespondstosthe transformer 'IB'ln Fig.3, and

The rcontrolxgrids :of. the valves A2 and V0 are 'ghtly coupled by -a small gcondenser Ci 6 for the purpose of supplying-ta quadrature input to the amplifyin valve;from-theioscillator. The anodes of: these :tww vahiesmrm connected. :togetherso that a controlling wave is applied to the oscillation circuit in quadrature with the oscillations. The amplification of the valve A2 is controlled by the signals which get through the gating valve. Any drift of the gating pulses produces a 2 compensating change through the variation in bias produced by the variation in amplitude of the pulses passing the gate in the manner previously explained, except for the lag produced by the rectifier circuit.

The anode load resistance RL may be inserted as before in series with the primary winding of the transformer T. The received pulses may be obtained if desired from terminal 5 connected to this resistance through a blocking condenser K,

or if RL is not provided they may be obtained from the anode at terminal 2 as in Fig. 2, for example.

In certain multi-channel pulse transmission systems a number of pulse trains all having the same repetition frequency are transmitted simultaneously, but are spread out in time so that all the pulses of all the trains occur at different times. Such trains are sometimes separately identified by imposing on each train a different modulating wave, such as a sine wave having a particular frequency designating the train, which may be conveniently called the identifying tone. Any type of modulation may be used, such as amplitude modulation, duration modulation or time phase modulation, or a combination of such modulations.

The identifying tone may be added to the modulating intelligence wave, for example in the case of a telephone channel, the identifying tone may be sine wave at 50 pzs or any other subaudible frequency. Alternatively, in the case of a pulse distribution system, it may be desired to reserve one channel for identification of phase only, and to use the synchronised receiving system for developing suitable gating or demodulating waves for the other channels.

The arrangement of Fig. 11 may be employed for picking out a particular one of such pulse trains having a given identifying tone. The only modification is that the transformer T and the coil 3 are tuned by their respective condensers to the frequency of the indentifying tone instead of to the repetition frequency of the pulses. Assuming that the valve V0 is arranged to oscillate at a slightly lower frequency than the pulse repetition frequency, then the gating pulses first drift continuously so that their phase is retarded with respect to the incoming pulses,

until they coincide with them, and amplified pulses then appear in the anode circuit of the gating valve V. If the signal pulses are amplitude or duration modulated, it is evident that the amplified pulses will be similarly modulated. If they are time-phase modulated they will occur at a varying position up and down the trailing edge of the gating pulses. Evidently, therefore, the pulses appearing in the anode circuit of the valve V will in all cases be amplitude modulated with the identifying tone. The modulating component is extracted by the tuned transformer and is rectified by the circuit Q, RIB, Clo as before to control the amplifying valve A2. It will be evident that the amplitude of the modulating tone which appears in the anode circuit of valve V will go through variations similar to those of the amplitude of the pulses as the phase of the gating pulse varies; and the operation of the circuit is the same as previously described, except that the frequency of the controlling wave is that of the identifying tone instead of the pulse repetition frequency. Thus when a number of pulse trains are present all with the same repetition frequency, only that one which has the identifying tone to which the transformer T is tuned can be picked up, since only that particular train is able to operate the amplifying valve A2. The pulses bearing the intelligence modulation which have passed through the gate may be extracted at terminal 5, or the pulses may be received and demodulated in a separate circuit (not shown) controlled by gating pulses from terminal 4, for example.

Fig. 12 is a modification of Fig. 7 employing a difierential rectifier of the kind described in connection with Fig. 9. A different type of oscillator controlled by a fork has been shown, but any other suitable type could have been used.

The two amplifying valves Al and A2 have their anodes connected in parallel to the input coil LI of the oscillation circuit which is tuned to the pulse repetition frequency by the condenser CH. The control rid of the oscillator valve V0 is coupled lightly to the control grids of Al and A2 in opposition through a small condenser C16 in series with the primary winding of a transformer TE of which the secondary winding is tuned to the pulse repetition frequency by the condenser CIS. This provides a quadra ture drive to these valves from the oscillator. When the pulses are being normally received, the control grid bias of both the valves Al and A2 will be zero, so they will produce a zero resultant output, and therefore there will be no controlling wave applied to the oscillator. When a drift occurs, the voltages applied by the gating valve to the rectifier circuit will cease to cancel out and the control grids of Al and A2 will eventually become biassed in opposite directions, thus applying to the oscillator a quadrature controlling wave adapted to correct the drift.

Fig. 13 shows an arrangement of similar type to Fig. 12 but adapted to select a pulse train designated by an indicating tone in a multichannel system of the kind described with reference to Fig. 11. It differs from Fig. 12 in that the oscillating circuit associated with the valve V0 is of the kind shown in Fig. 11, and is connected to the transformer TE through a winding L4 closely coupled to the winding L0 instead of through a condenser such as C16. These diiferences are immaterial, and the oscillator and coupling arrangement of Fig. 12 could have been used in Fig. 13.

The differential rectifier circuit used in this case for controlling the amplifying waves AI and A2 comprises substantially two rectifier circuits like that of Fig. 11 arranged in opposition, connected respectively through resistances RNA and RMB to the control grids of valves Al and A2. The components of these two rectifier circuits are given the same designations as those of the rectifier circuits in Fig. 11 with the addition of the letters A and B respectively. The resistances RIZA and RIZB, each shunted by a by-pass condenser, provide the grounded centretapped load for the pair of rectifiers.

The anodes of the gating valves are connected through the primary windings of the transformers TA and TB to the terminals of the primary winding of a transformer TF, the centre point er which is connected through the primary winding of atransformer TG to the terminal HT+. The secondary winding of trans 17 formers TA and'TB are tuned to the frequency of the identifying tone by the condensers A and 05B and are connected respectively to the input sides of the two rectifiers.

It will be understood from the explanations already given in connection with Figs. 11 and 12 that the gating pulses will drift in phase until the incoming pulse train bearing the identifying tone is picked up, and the identifying tone is then selected by the transformers TA and TB and rectified and applied to the control grids of the valve Al and A2. So long as the incoming pulses are symmetrically meshed between the two series of gating pulses, the control grid voltages of the two valves Al and A2 will be equal. If a drift occurs whereby the pulses tend rather to pass through VA and VB, then the grid voltage of Al is raised and that of A2 is lowered and a corrective force is thereby applied in the manner already explained. Similarly an opposite drift is connected in the reverse manner.

The transformers TF and TG are provided for extracting the modulation from pulses which have passed through the gates. Transformer TF being connected differentially will be used when the pulses are time phase modulated, because in that case the outputs of the two valves will be continually varying in opposition, on account of the modulation. It will be understood that the lag in the rectifier circuit will prevent the gating pulses from following the rapid variation of the time phase modulation.

When the pulses are duration or amplitude modulated, the transformer TG is used, since in that case the outputs of both valves vary in the same direction with the modulation. Clearly,

if only one type ofmodulation is involved, one of the transformers TF or TG will not be wanted and may be omitted.

In all the embodiments which have been described so far, frequency division of the incoming pulse train may be obtained by tuning the filter or oscillator circuit to a subharmonic of the incoming pulse train. The repetition frequency of the gating pulses will then be equal to say l/nth of that of the incoming pulses and the gate or gates, will accordingly be opened to admit only every nth pulse of the train. The arrangement then operates as if all the other pulses in the incoming train were absent.

The arrangements which have been described with reference to Figs. 8 to 13 included a rectifier circuit of high time constant for the purpose of introducing a lag for preventing hunting. When other pulse trains have repetition frequencies which are very close to that of the wanted train, or when there is much interference, it is necessary to use a very highly selective filter or very stable oscillator for the device SWG, in order to avoid picking up the wrong pulse train, or to suppress the interference. In either case a tuned circuit or other resonator of very low decrement must be used, such as an electromagnetically driven fork, or a piezoelectric crystal, for example. Such a low decrement resonating system is a predisposing cause to hunting because there is implied a lagging response to the corrective or controlling drive which encourages the hunting.

It has been found that this predisposition to hunting can be greatly minimised by providing bourhood of resonance, so

a small aperiodic leak across the resonant ele- 'ment. This modifies only imperceptibly the response curve of the element near the resonance point, but provides a small response, free from inertia, to sudden changes of excitation. The

pletely avoided. It is clear,

effect of a sudden change of drive is'to produce a small but instantaneous change of output, which then changes very slowly by a further amount while the resonant element takes up the new steady state. Sufficient-leakage should be provided so that an adequate range of quick response is obtained to suppress the hunting.

Fig. 14 shows one simple way in which this may be done. In this figure SF is a selective filter element such as an electrically controlled fork arranged, for example, as shown in'Fig.2,.or a piezoelectric crystal or any. other resonant system of low decrement which would very strongly attenuate waves of all frequencies except those very close to the resonance frequency. shunted across the input terminals IN of SF is one pair of diagonal terminals of a Wheatstone bridge comprising a centre tapped primary winding of the transformer TH, connected to which in parallel is an adjustable potentiometer P and a differential condenser CD. The secondary winding of TH is connected in series with the output terminals OUT of SF.

If the bridge is balanced, then substantially no effect is produced, but if it be unbalanced slightly by adjustment of P and/or CD a small amount of the input waves having any desired amplitude and phase will be shunted round the element SF. This provides the means for obtaining the desired aperiodic leak which is not affected by the selective action of the element. It will be noted that the bridge is connected in parallel on the input side and in series on the output side It will be evident'that it could be connected in series or in parallel at either end as desired.

It will be understood that in the absence of the bridge circuit there is likely to be a small amount of direct transfer of energy from the input to the output terminals due to capacity or other couplings which cannot always be comtherefore, that the bridge could be adjusted so that such direct transfer is exactly neutralised, and in that case the hunting tendency would be a maximum. Such an exact balance is not desired, and an appropriate unbalance is produced in the followtained. The potentiometer is then displaced in such a direction that the slight resultant leakage produced is in the same phase as the output of the resonant element. The exact balance first made by means of the condenser prevents any quadrature currents passing round the resonant element so-that the phase of the output remains constant while it is building up. If the resistance balance also remained exact the automatic phase control circuits of the various embodiments described tend to produce slow phase hunting believed to be due to the exceedingly sharp rate of change of phase with frequency in the neighthat the resonant element introduces a large delay or inertia. By providing a definite small leak in phase with the outwhich can be produced by the whole circuit is restricted thus preventing low speed hunting. If

.75 hunting or fiutter tends excessive, a fast phase to be produced owing to the leakage provided is the loss of selectivity resulting from the excessive leak. Generally, however, an adjustment can be found which greatly reduces any tendency to hunting.

Fig. 15 shows a bridge arrangement employing a piezoelectric crystal to form a selective filter with an aperiodic leak on the same principles as 14. The bridge comprises the two halves of the secondary winding of the transformer TH tuned by the variable condenser C22 and the two parts of the adjustable potentiometer P one of which is shunted by the crystal Z and the other by adjustable condenser CM. If the bridge is balanced at some frequency remote from the resonance frequency of the crystal, there will be practically no output except near the resonance frequency, at which the crystal effectively shortcircuits one arm of the bridge. The aperiodic leak is provided in the manner explained with reference to Fig. 1&1 by slight adjustment of the potentiometer away from the balance point.

In the case of either Fig. 14 or Fig. 15, condensers (not shown) may be provided, if necessary, across the in and out terminals to tune the input and output circuits of the device, and the arrangements may be adapted suitably to the nature of the selective element SF. Input and/or output amplifiers (not shown) may be provided if required. By connecting the input and output terminals through an appropriate amplifier, the circuit of Fig. 14 or 15 then forms an oscillator suitable for any of the embodiments described which employ an oscillator as the deviceSWG instead of a filter, in which the hunting tendency due to the selective element is obyiated by the aperiodic leak.

'The arrangement of Fig. l4 is particularly effective in the circuit of Fig. 12. In this circuit the fork oscillator cannot easily develop a fast flutter on account of the large time la introduced by the rectifier circuits. The controlled leak adjusted as described quenches the low frequency hunting swings which could be transmitted through the control circuits.

In the arrangements of Figs. 1 to 13, the range of frequency variation over which synchronisation can be maintained depends on the range of phase control which can be provided, and on the extent of variations of the change of phase through the selective element. The range of control can therefore be extended if the variation of phase change through the element can be eliminate.

Fig. 16 shows an amplifying selective circuit including a fork resonant element which provides the same selectivity as the simple electrically driven fork device SWG of Fig. 2, but without the frequency variation of phase change which is inherent in the simple arrangement. Fig. 17 shows a modification of the part of Fig. 16 enclosed in the dotted lines to enable a piezoelectric crystal circuit of the kind shown in Fig. 15 to be used. The circuit can obviously be suitably modified to employ other types of resonant element without affectin its operation in principle. Corresponding leads leaving the dotted outlines in the two figures are designated with the same small letters a to 1.

Referring first to Fig. 16, the dotted outline shows an electrically driven fork F arranged in substantially the same way as, shown in Fig. 2 except that a coil L is loosely coupled to LI, and an optional condenser Cl! is shown to tune the input circuit.

The output of the fork circuit is arranged to drive in parallel the control grids of two variable gain amplifying valves AI and A2, but by means of the suitably chosen resistances and condensers RIB, Cl8 and RIB, Cl9 the valve Al is driven with the phase retarded by 45 and A2 with the phase advanced by 45. The anodes of the two valves are connected in parallel through the primary winding of the output transformer TJ to the positive high tension terminal HT+.

The cathodes of the valves are biassed for normal amplification by means of the conven tional condenser-resistance circuit shown, and the control grids derive variable bias from the bridge rectifier circuit comprising the rectifiers QA and QB, resistances RIOA and RIOB and condensers 010A and CIBB arranged substantially as in Fig. 13. Resistances RZDA and R203 are load resistances for the bias circuit. Bypass condensers K connected to earth are provided for the two control grid circuits.

The rectifiers QA and QB are driven in opposition from a transformer TK comprising a loosely coupled primary winding 46 connected to L5 and a centre tapped secondary winding Ll. They are also driven in parallel from the combined output of the valves Al and A2 through the connection between the secondary winding S2 of the t transformer TJ and the diagonal points of the bridge rectifier circuit.

The coils LI and L2, and the transformers TJ and TK are all tuned by their respective associated condensers CIT, C4, C2l and 020 to the pulse repetition frequency. When the valves A I and A2 have equal bias, the voltage across TJ will be in phase with the output from the fork F across the coil L2, and this is in phase with the input voltage at the terminals IN. Thus since the two voltages applied to the rectifier circuit by the transformers TJ and TK are in quadrature, there will be no difference of potential between the points A and B, and the control grids of both valves Al and A2 will be at the same potential.

Now let it be assumed that the pulse repetition frequency increases slightly. This causes a retardation of the phase of the voltage across TJ so that a greater voltage is applied to QB and to QA. The point B therefore, becomes positive to A, increasing the gain of A2 and decreasing thatof Al. This slightly advances the phase of the voltage applied to TJ so that the change is compensated. Evidently the reverse effect would occur if the pulse repetition frequency were to decrease. Thus the output obtained at the terminals OUT connected to the secondary winding of the transformer TJ will be always maintained substantially in phase with the input at the terminals IN irrespective of the phase changes introduced by the fork system as a result of changes in the input frequency. It will be clear that phase correction will occur for any change resulting in a difference between the resonance frequency of the fork and the input frequency.

Similar results will be obtained whatever the nature of the resonant system inside the dotted outline. Thus a filter of the kind shown in Fig. 15 may be used, and could be arranged, for example as shown in Fig. 17, which may be substituted for the circuit in the dotted outline of Fig. 16. A coil L5 is loosely coupled to the transformer TH for supplying the quadrature connection to the transformer TK in Fig. 16, and the inductance coil L8 is provided in the position of L2, but is used as a choke to prevent the output 

